[sdiy] Xilinx 3E board comments

[Eric Brombaugh]

Paul Maddox wrote:
> the monowave has an update rate of 1Mhz, so a nyquest of 500khz, but 
> you need to get above 4Khz pitch before you can begin to hear the 
> artifacts.. but by then a 4Khz note is begining to hurt my ears...
> Given that few people would play a note above 6Khz for any length of 
> time, I reckon a 50Mhz update rate is as good as 'alias free' as 
> you'll ever get.
That's good info. Lacking a lot of practical synth experience I don't 
yet have the rules-of-thumb that define the fuzzy line between 
'overkill' and 'good enough'.

That said, a 6kHz sawtooth generated at a 50MHz sample rate will have 
worst-case aliases in the -78dB range. Those will be barely audible in a 
mix of any complexity. Additionally, if you don't have at least 13 bits 
of linearity in your DAC they'll get swamped out by quantization noise 
and THD. Consequently, the 12-bit DAC on the 3E Starter Kit won't even 
resolve them.

Eric